1. Field of the Invention
This invention relates to an improved method for preparing alkyl methacrylates from alkyl isobutyrates by oxydehydrogenation of the latter over an oxydehydrogenation catalyst in the presence of molecular oxygen wherein an amine is also included in the reaction mixture.
2. Description of the Prior Art
There exists considerable prior art relating to the oxydehydrogenation of the lower saturated aliphatic monocarboxylic acids to produce the corresponding, .alpha.,.beta.-olefinically unsaturated acids. Early work in this area involved thermal, vapor phase oxydehydrogenation of the saturated aliphatic carboxylic acid in the presence of oxygen and iodine. This approach has not been particularly successful from a commercial standpoint. This is understandably so inasmuch as iodine is costly, exhibits extreme corrosivity properties and poses considerable problems in realizing complete recovery of the comparatively large amounts thereof required in the process. The heterogeneous catalytic method for oxydehydrogenation according to the prior art appears to be the more attractive route to the commercial production of .alpha.,.beta.-olefinically unsaturated monocarboxylic acids. The prior art heterogeneous oxydehydrogenation catalysts useful for this purpose include some heteropoly acids, such as phosphomolybdic acid, optionally with tungsten and/or vanadium. Another type of catalyst included in the prior art is iron phosphate.
Iron phosphate subjected to calcination exists in several crystalline phases or species. It is not known at this time which species is or are catalytically active. There is evidence that the presence of certain extrinsic metal components in the catalyst preparation serves to facilitate the formation of the active catalyst. For instance, U.S. Pat. No. 3,948,959 discloses that an alkali or alkaline earth metal can be the extrinsic metal for this purpose. Similarly, other catalysts useful for the oxydehydrogenation reaction are disclosed in U.S. Pat. Nos. 3,917,673; 3,855,279; 4,088,602 and 3,634,494 and elsewhere.
None of the prior art discloses or suggests that the inclusion of an amine in the oxydehydrogenation feed will give much improved conversion and selectivity in the transformation of an isobutyric acid ester to the corresponding methacrylic acid ester.